FIG. 1 illustrates in perspective view a prior art wind turbine blade 10. The wind turbine blade 10 extends longitudinally from a generally cylindrical root 11 to a tip 13. In use, the root end 11 of the blade 10 is attached to a hub of a wind turbine (not shown). In cross-section, the blade 10 transitions from a circular profile at the root 11 to an airfoil profile at the widest part of the blade 10, which is known as the ‘shoulder’ 15. Between the shoulder 15 and the tip 13, the blade has an airfoil profile that steadily decreases in thickness and chord moving towards the tip 13.
FIG. 2 illustrates the blade 10 in cross-section and reveals that the blade 10 comprises an outer shell 12, which is fabricated from two half shells: a leeward shell 14 and a windward shell 16. The shells 14, 16 are moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 12 are of sandwich panel construction and comprise a core 18 of lightweight foam (e.g. polyurethane), which is sandwiched between inner and outer GRP layers 20, 22 or ‘skins’.
The blade 10 comprises first and second pairs of spar caps 24, 26, 28, 30 arranged between sandwich panel regions of the outer shell 12. One spar cap of each pair 24, 28 is integrated with the windward shell 16 and the other spar cap of each pair 26, 30 is integrated with the leeward shell 14. The spar caps 24, 26, 28, 30 of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10. A first longitudinally-extending shear web 32 bridges the first pair of spar caps 24, 26 and a second longitudinally-extending shear web 34 bridges the second pair of spar caps 28, 30. The shear webs 32, 34 in combination with the spar caps 24, 26, 28, 30 form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine (not shown). The spar caps 24, 26, 28, 30 in particular transfer tensile and compressive bending loads, whilst the shear webs 32, 34 transfer shear stresses in the blade 10.
The shear webs 32, 34 are made in a vacuum-assisted resin transfer moulding process (VARTM). The process is illustrated in FIG. 3, which shows only a bottom part of the web. Layers of glass fibre fabric 40 are laid up on the surface of a mould tool 50. Edges 42 of the layers are aligned to define an edge 43 of the shear web 32, 34. The layers 40 are enclosed by a vacuum bag 46 to define a sealed area around the layers 40. Air is then removed from the bag 46 by a vacuum pump, and resin is introduced into the bag 46. The resin infuses between and into the glass fibre layers 40. The mould tool 50 is then heated to cure the resin, and once the resin is cured, the component is demoulded.
The resulting shear web 32, 34 has an edge 43 that is defined by the aligned edges 42 of the glass fibre layers 40 and the cured resin in the region of those edges 42. It is difficult to align the edges 42 accurately when the layers 40 are laid on the mould tool 50, and so the edge 43 tends to be uneven. Furthermore, uneven amounts of excess resin tend to be present at the edge 43 of the shear web 32, 34 after the resin infusion. The result is that after the resin has been cured, the edge 43 of the shear web 32, 34 tends to be rough, with the hard cured resin at the edge 43 forming serrations. The rough edge 43 may therefore present a safety hazard making the shear web 32, 34 difficult and potentially unsafe to handle.
The potentially hazardous rough edge 43 can be removed post-fabrication by trimming the edge of the component. However, the cured resin is very hard, and a diamond cutting saw is required to cut through the component to trim the edge, making the trimming an expensive and time-consuming process.
The edge 43 can be neatened prior to the curing process by folding over edges of the glass fibre layers 40 when they are laid in the mould tool 50; however this only partially mitigates the problem, and in practice trimming is still required post-fabrication. The edge 43 can also be neatened by providing the mould tool with an abutment piece in the form of a metal bar or strip, and laying the glass fibre layers 40 against the abutment piece. When the component is de-moulded, the resulting edge is neater. However, even in this case the roughness remain to some extent, and the act of de-moulding the component still tends to leave fractured regions of resin that present sharp and hazardous edges.